In a recording and reproducing system which employs, as a recording medium, an optical disc such as a CD (Compact Disc), what is called an MD which is a small magneto-optical disc with a diameter of 64 mm, a DVD (Digital Versatile Disc) or the like, a device has been hitherto demanded to be more compact, lighter and more inexpensive. In order to meet such a demand, individual component parts need to be compact and light and inexpensively manufactured. An optical pick-up device for the above-described optical disc realizes a compact, light and further inexpensive structure of the device itself by integrating a semiconductor laser serving as a light source and a photodetector such as a Pin diode composed of silicon (Si) as a base material.
The photodetector used for the optical pick-up device is formed as what is called a “PDIC” (photodetector device) composed of one chip in which is incorporated an integrated circuit for performing processes and calculation of a light receiving signal such as receiving light reflected from the optical disc, converting the light into an electric signal and outputting the signal to integrate functions for receiving light, calculating the received light, converting the received light into an electric signal and outputting the electric signal, so that the compact, light and further inexpensive optical pick-up device using the photodetector is realized.
Here, elements capable of being integrated and compact by adding optical parts such as a prism, a lens, a diffraction grating, a hologram, etc. to the light source and the photodetector forming the optical pick-up device are generally referred to as hybrid optical elements. As shown in FIG. 1, the hybrid optical element includes a semiconductor laser 101, a PDIC 102 and optical elements such as lenses, prisms, diffraction gratings, holograms, etc. arranged on a substrate 104 which are electrically or mechanically connected together.
The semiconductor laser 101 is attached to a sub-mount 101a by a chip mount step. The sub-mount 101a is attached to the PDIC 102 by a die bonding step. At this time, in the sub-mount 101a, connection terminals are electrically connected to wiring pads of a PDIC substrate 102a forming the PDIC 102. On the PDIC substrate 102a, markers 102c showing positions in which the sub-mounts 101a are arranged are provided. The sub-mount 101a has its attached positions positioned to the PDIC substrate 102a and is attached to the PDIC substrate 102a in such a manner that markers 101b provided on the sub-mount 101a itself are made to correspond to the markers 102c. 
To the PDIC substrate 102a of the PDIC 102, photodetectors 102b are attached. On the PDIC 102, a prism 103 is attached by a prism mount step. The PDIC 102 is attached on a substrate 104 by a PDIC mount step.
In manufacturing the hybrid optical element, markers 104a as the marks of positions are further previously provided on the substrate 104 and the respective parts are attached to the substrate 104 by using these markers 104a as references for positioning. The above-described positioning method is referred to as a “passive alignment”. The accuracy of the hybrid optical element is determined by the accuracy of a positioning (alignment) step of each parts relative to the substrate.
In the substrate 104, electrodes 104b to which the PDIC 102 is electrically connected are provided. The PDIC 102 is electrically connected to the electrodes 104b by a wire bonding step as shown by arrows in FIG. 1.
The parts respectively forming the hybrid optical element are basically arranged as shown in FIG. 2. That is, the semiconductor laser (light source) 101, a signal recording surface of an optical disc 107 and a light receiving part of the photodetector 102b are respectively arranged to be located at positions of image points through the reflection surface of the prism 103. The positions of the respective parts are arranged so as to deviate from other positions.
In other words, a light flux outgoing from the semiconductor laser 101 is a diffusion light flux that is light having only a semiconductor laser side as a focal point. The light flux outgoing from the semiconductor laser 101 is allowed to be a parallel light flux by a collimator lens 105 and converged on the signal recording surface of the optical disc 107 by an objective lens 106. At this time, the objective lens 106 is controlled to move in a focusing direction parallel to an optical axis so that a focal point is always formed on the signal recording surface of the optical disc 107.
The light applied to the signal recording surface of the optical disc 107 is reflected on the signal recording surface and converged on lenses 108 and 109 through the objective lens 106, the collimator lens 105 and the prism 103 and returned to the photodetector 102b. The optical elements are respectively arranged so that the reflected light from the signal recording surface of the optical disc 107 focuses on the light receiving part of the photodetector 102b. That is, the light is located respectively at the focal points in the semiconductor laser 101, the signal recording surface of the optical disc 107 and the light receiving part of the photodetector 102b. This means that the light source, the signal recording surface and the light receiving part are located at the positions of image points.
On the other hand, for more inexpensively providing the hybrid optical element, the number of parts is devised to decrease as many as possible. For instance, as shown in FIG. 3, in the structure that lights are returned respectively to a plurality of light receiving parts of the photodetector 102b having the optical path lengths from the signal recording surface of the optical disc different from each other, one lens 110 is used as a lens for respectively providing the light fluxes in the light receiving parts as focal points on the light receiving parts.
Since the optical distances to the light receiving parts from the lens 110 are precisely different from each other, the hybrid optical element does not obtain the focal points at the same time. However, a focus error signal, a tracking error signal, a reflection signal (RF signal) based on the reflected light flux read from the optical disc or the like may be obtained so as to be allowable in practice. A step for reducing and adjusting the number of optical parts is simplified as mentioned above, so that the compact and inexpensive hybrid optical element can be realized.
The wavelength of light emitted from a light source used for the optical pick-up device is shortened as well as the versatility of recording media such as the optical disc.
For meeting the versatility of the recording media, there are proposed optical discs such as a ROM optical disc in which an information signal is formed on a disk substrate by a micro irregular pit pattern, a phase change optical disc having a phase change recording layer in which an information signal can be recorded and reproduced, a magneto-optical disc having a magneto-optical recording layer in which an information signal can be recorded and reproduced, etc.
In the ROM optical disc and the phase change optical disc, there are provided areas having different reflectance depending on the information signals recorded on the optical disc and the change of light intensity due to the difference in reflectance of light fluxes reflected from the areas is detected to read the information signals recorded thereon. The magneto-optical disc uses a magneto-optical Kerr effect in the magneto-optical recording layer and reads the information signals recorded based on the difference in polarizing angle of the reflected lights.
The optical pick-up devices used for reading the information signals from these optical discs are also requested to satisfy respective systems so as to meet the optical discs having the above-described versatile recording systems. In the optical pick-up devices, the structures of the hybrid optical elements to be used are respectively different so as to meet the optical discs respectively having different recording systems.
For instance, the photodetector 102b of the optical pick-up device used for reading the information signals from the ROM optical disc on which the information signals are recorded by the pit pattern may be provided with, as shown in FIG. 4, one four-divided light receiving part 111 for detecting a focus error signal and an RF signal and two light receiving parts 112 and 113 for detecting a tracking error signal which are arranged at positions to sandwich the four-divided light receiving part 111 in between them. The structure of the optical element for making the light reflected from the optical disc incident on these light receiving parts is also simple. As compared therewith, the photodetector 102b of the optical pick-up device employed for reading the information signals recorded on the magneto-optical disc needs, as shown in FIG. 5, light receiving parts 114 and 115 for detecting light intensity for each of different polarized states in addition to one four-divided light receiving part 111 for detecting a focus error signal and an RF signal and two light receiving parts 112 and 113 for detecting a tracking error signal. That is, since a magneto-optical signal is feeble, a differential detection needs to be carried out by using the two light receiving parts 114 and 115 on the photodetector 102b. As described above, the number of the light receiving parts is increased so that the structure of the optical element for making the light reflected from the optical disc incident on these light receiving parts is likewise complicated. In other words, in order to make the light reflected from the optical disc incident on these light receiving parts, the reflected light needs to be branched into three light fluxes and at least two prisms are required. At this time, as for the change of polarizing direction due to the magneto-optical Kerr effect, polarized waves are divided into P-polarized waves and S-polarized waves relative to the reflecting surfaces of the prisms. The S-polarized wave is reflected on the reflecting surface of one prism to allow it to be incident on one light receiving part 114 of the photodetector 102b and the P-polarized wave is reflected on the reflecting surface of the other prism to allow it to be incident on the other light receiving part 115 of the photodetector 102b. Thus, the change of the polarizing direction can be detected. As for the reflected light when there is no information signal recorded on the optical disc, the arrangement of the hybrid optical element needs to be determined so that the intensity of the S-polarized wave is equal to that of the P-polarized wave or the optical elements such as a ½ wavelength plate need to be used. As described above, to meet the various kinds of optical discs having different recording systems, the structure of the hybrid optical element having the optical pick-up device is complicated.
On the other hand, in order to meet the short wavelength of light emitted from the light source, the index of refraction and the angle of diffraction of the optical elements forming the optical pick-up device cause problems. For instance, for the optical pick-up device used for reading the information signals of the optical disc such as a CD on which the information signals are recorded by the pit pattern or the magneto-optical disc such as an MD, a light source for emitting light whose wavelength is 780 nm is employed. For the optical pick-up device used for a DVD, a light source for emitting light whose wavelength is 650 nm is employed. Further, for the optical pick-up device used for the optical disc capable of performing a high density recording, a light source for emitting light whose wavelength is 450 nm is employed.
When the index of refraction of the optical elements employed for the optical pick-up device is examined, its imaginary part, that is, its absorption is firstly apprehended. In optical glass or a synthetic resin material used as the material of the optical elements of the optical pick-up device, light having the wavelength shorter than about 400 nm is greatly absorbed. When such absorption arises, the output of emitted light of the light source needs to be more increased in order to obtain necessary reflected light. Further, since the change in quality of the optical elements is generated, the degree of freedom in selecting the materials of the optical elements is restricted. As the wavelength of a light flux becomes short, the angle of diffraction becomes small. For instance, assuming that the pitch of the diffraction grating is d, the angle of diffraction θ in this diffraction grating is represented as described below.sin θ=mλ/(nd)  (1) (Here, m designates an integer, 1 designates wavelength and n designates index of refraction.)
According to this formula (1), as the wavelength of the light flux becomes short and λ becomes small, the angle of diffraction θ becomes small under a condition that the pitch d and the index of refraction n are constant.
The plural light receiving parts formed on the PDIC need to be formed to have respective sizes and spaces so that photodetection signals corresponding to light fluxes respectively incident on the light receiving parts can be independently detected. Each size and space are determined depending on the property of the photodetector and a capability of a manufacturing step, and do not directly undergo a limitation of the wavelength of the light flux. That is, it may be said that the size and space of each light receiving part on the photodetector are determined independently of the wavelength of light emitted from the light source. Accordingly, the size and space in which each light receiving part can independently detect the photodetection signal may be considered not to depend on the wavelength of light emitted from the light source and to be constant.
In this case, according to the above-described formula (1), an optical distance to each light receiving part from the diffraction grating needs to be lengthened in order to distribute the reflected light to and receive it by each light receiving part, so that the structure may be possibly enlarged. In this case, as mentioned above, even when the focus positions of the reflected light are displaced from the position of the photodetector within an allowable range to make it easy to form the hybrid optical element, the allowable range is extremely narrowed. In the present case, the structure that the reflected light is converged on a plurality of light receiving parts by one lens as described above cannot be employed.
As described above, when the method for using the markers as positioning marks to position the optical elements as mounting means is employed, it is difficult to obtain a good positioning accuracy due to a fact that the optical distance is long and an adequate positioning cannot be realized. When the optical distance to the light receiving part from the diffraction grating increases twice for an angle setting accuracy with the same accuracy, the positional displacement between the light receiving part and a spot of light will become twice. The above-described positional displacement between the light receiving part and the spot of light causes the quality of the photodetection signal which is to be detected to be deteriorated. In the worst case, it is anticipated that the photodetection signal is not detected.
As mentioned above, in the hybrid optical element forming the optical pick-up device, a signal detecting system is complicated with the versatility of the optical discs as well as the improvement of the recording density, and further, a high accuracy in assembly is required.
For satisfying such a request, a double sided mounting is carried out so that the optical elements such as a lens, a prism, a diffraction optical element, etc. are attached to one surface of a substrate serving as a base and a semiconductor laser serving as a light source and photodetectors are attached to the other surface of the substrate. Further, in an assembly step, while viewing the state of the photodetection signal, a positioning (active alignment) is carried out. In order to realize the above-described structure and the assembly step, the substrate, the optical elements, the semiconductor laser and the photodetector need to be formed in configurations adapted to such structure and assembly step. Firstly, as shown in FIG. 6, it is necessary to form a light transmission hole 116 on a substrate 104 so that the light receiving part 102c of a photodetector 102b can detect the reflected light reflected by the optical disc through the light transmission hole 116. The photodetector 102b needs to be attached to the substrate 104 so as to oppose the light receiving part 102c to the substrate 104.
An electric signal detected in accordance with the reflected light received by the photodetector 102b needs to be taken outside the package of the hybrid optical element. As a first structure to this end, there is a structure that wiring is provided on a substrate 104 and a terminals of photodetectors are directly connected to the wiring.
As a second structure, for example, as described in Japanese Patent Application Laid-Open No. 2000-228534 and Japanese Patent Application Laid-Open No. 2000-183368, there is a structure that a relay substrate through which electrodes are taken out is interposed between photodetectors and a substrate. In the Japanese Patent Application Laid-Open No. 2000-228534, there is disclosed a structure that the photodetectors are connected to the relay substrate by an anisotropic conductive material. In the Japanese Patent Application Laid-Open No. 2000-183368, there is disclosed a structure that optical elements are attached to one surface side of the relay substrate and the photodetectors are attached to the other surface side by a flip chip bonding method.
As a third structure, photodetectors are used having a structure that light receiving parts and electrode terminals are formed on opposite surfaces to each other.
However, the conventionally used photodetectors have structures that the light receiving parts and the electrode terminals are provided on the same surface and do not have structures that the light receiving part and the electrode terminals are respectively arranged on both the surfaces of one surface and the other surface.
The conventional hybrid optical element utilizes a structure that when the photodetectors are sealed in package (PKG), the chips of the photodetectors are mounted on a resin mold package and a wire bonding process is applied thereto to form a light receiving side with a molded resin, or a structure that the chips of the photodetectors are mounted on a hollow package made of a molded resin or ceramics, a wire bonding process is applied thereto, and the package is covered with a flat plate made of a glass substrate or a synthetic resin.
As described above, the hybrid optical element and the PDIC (photodetector device) have the structure of the signal detecting system complicated, need to have a high accuracy in assembly and not to carry out the conventional passive alignment, but to carry out the active alignment.
As the structures in which the active alignment can be carried out, it may be said that the third structure is the simplest among the first to third structures. In the third structure, as shown in FIGS. 7A to 7C, a photodetector 102b is bonded to a substrate 104 by, for instance, a UV (ultraviolet ray) curing resin so that the light receiving surface of the photodetector 102b comes into contact with the substrate 104 and electrodes for taking out signals are formed on a surface opposite to the light receiving surface so that the signals can be taken outside a package.
For manufacturing the photodetector having the above-described structure, there exist some technical problems to be solved. As one of the problems, there may be considered a formation of a light transmission hole. In the photodetector, the light transmission hole 116 having the depth not less than the thickness of the substrate 104 needs to be provided. Accordingly, there may be considered problems such as the relation between the opening area of the light transmission hole 116 and the size of the photodetector 102b, or an insulating part to be provided on the inner peripheral surface of the light transmission hole 116 and the disconnection and reliability of an electrode material to be inserted. Therefore, the photodetector having such a configuration is hardly formed.
The photodetector 102b disposed on the substrate 104 is, as shown in FIG. 7C, electrically connected to the substrate 104 by wire-bonding between wire bonding pads 104c provided on the substrate 104 and wire bonding pads 102d provided on the photodetector 102b. 
On the other hand, in the first structure, that is, the structure that the photodetectors having the light receiving parts and the electrode parts formed on the same surface are directly bonded to the substrate by a flip chip bonding process, it is difficult to carry out the active alignment in which an assembly and a positional adjustment are performed while viewing photodetection signals from the photodetectors.
In order to output the photodetection signal from the photodetector during a positioning operation while using the first structure, for instance, as shown in FIG. 7, a conductor such as a probe pin 117 needs to be brought into contact with the electrode part of the photodetector 102b. That is, when the active alignment is carried out in the first system, a space into which the probe pin 117 is adequately inserted needs to be provided between the photodetector 102b and the substrate 104 while the position of the photodetector 102b is adjusted. Under a state that the substrate 104 is kept coming into contact with the photodetector 102b, the position of the photodetector 102b cannot be adjusted. When the space is provided between the photodetector 102b and the substrate 104 during adjusting the position of the photodetector 102b, a spot size on the light receiving surface upon adjustment of the position is different from a spot size after the assembly. Therefore, an output signal which is precisely obtained upon adjustment changes after the assembly so that a precise positional adjustment is difficult.
It is extremely difficult to employ the first to third structures for the hybrid optical element in which at least one of optical elements a lens, prism, a diffraction element, etc., a light emitting element, and photodetectors are mounted on both the surfaces of the substrate from the viewpoints as mentioned above.
When the photodetectors are sealed in a package (PKG), when a light source such as a violet blue laser which emits light whose wavelength is short, for example, about 400 nm is used, most of molded resins absorb the light located in this wavelength band, so that a synthetic resin material which has been hitherto employed for infrared rays or visible lights (red to blue) cannot be used.
That is, in the structure that the chips of the photodetectors are mounted on the resin mold package and the wire bonding process is applied thereto to form the light receiving side by the molded resin, the molded resin having a high transmission factor in a short wavelength band needs to be used, however, when a moldability and sealing characteristics or the like are taken into consideration, there is no proper material. In the structure that the chips of the photodetectors are mounted on the hollow package made of the molded resin or ceramics, the wire bonding process is applied thereto, and then, they are covered with the flat plate made of the glass substrate or the synthetic resin or the like, not only does a manufacturing cost becomes high, but also an entire size is enlarged.